A Kinetic Signature for Parallel Pathways: Conformational Selection and Induced Fit. Links and Disconnects between Observed Relaxation Rates and Fractional Equilibrium Flux under Pseudo-First-Order Conditions

Molecular association plays a ubiquitous role in biochemistry and is often accompanied by conformational exchange in one or both binding partners. Traditionally, two limiting mechanisms are considered for the association of two molecules. In a conformational selection (CS) mechanism, a ligand preferentially binds to a subset of conformations in its binding partner. In contrast, an induced fit (IF) mechanism describes the ligand-dependent isomerization of the binding partner in which binding occurs prior to conformational exchange. Measurements of the ligand concentration dependence of observed rates of relaxation are commonly used to probe whether CS or IF is taking place. Here we consider a four-state thermodynamic cycle subject to detailed balance and demonstrate the existence of a relatively unexplored class of kinetic signatures where an initial decrease in the observed rate is followed by a subsequent increase under pseudo-first-order conditions. We elucidate regions of rate space necessary to generate a nonmonotonic observed rate and show that, under certain conditions, the position of the minimum of the observed rate correlates with a transition in equilibrium flux between CS and IF pathways. Furthermore, we demonstrate that monotonic trends in the observed rate can occur when both CS and IF mechanisms are taking place, suggesting that caution must be taken not to overinterpret monotonic trends as evidence of the absence of either CS or IF. Lastly, we conclude that a nonmonotonic kinetic signature is uniquely unambiguous in the sense that when this trend is observed, one may conclude that both CS and IF mechanistic paths are utilized.